The present invention relates to atmospheric measuring instruments. More particularly, it relates to an attachment for mechanical recording type meteorological instruments which permits conversion of instruments originally constructed only to record a measurement to an instrument which will also provide an electrical alarm signal whenever a predetermined limit of the measurement is reached.
A typical recording instrument to which the present invention is applicable is the hygrothermograph. This instrument comprises a revolving drum driven by a clockwork mechanism. A recording chart having the upper portion divided into degrees of temperature and the lower portion divided into percentages of relative humidity is mounted on the drum to be carried beneath a pair of pen arms, one of which is controlled by a temperature responsive sensor and the other of which is controlled by a sensor responsive to relative humidity. Such an instrument provides a continuous record, over a period of time dependent upon the drum speed, of the temperature and relative humidity of the air to which the instrument is exposed. The instrument is entirely mechanical in operation and provides no electrical signal indicative of the conditions being measured. There are many hundreds of recording instruments of the general design of the hygrothermograph in use for monitoring air conditions in living and working quarters, museums, factories, warehouses and the like. If it is necessary to control the condition of the air within certain limits it is necessary to maintain a more or less continuous watch on the instrument indication so that appropriate adjustments can be made in the heating or airconditioning system of the building in which the instrument is located. Often such buildings are left unattended for extended periods and in the absence of proper monitoring, conditions can get out of hand and extensive damage can result.
The inclusion of limit switches for actuating electrical alarms is a relatively straight forward matter if such a requirement is considered from the beginning of the design of an instrument. When it is desired to add an alarm feature to instruments of established design and to do so in a manner which will not require factory rebuilding of the instrument, the provision of an alarm feature becomes more difficult. A satisfactory assembly for adding an alarm to existing instruments is one which can be installed by the user without the need for special tools, one which will not affect the accuracy of the instrument nor interfere with its basic function of measuring and recording a particular physical quality or condition, one which provides both high and low alarm set points, and one in which the alarm set points can be conveniently set by the user.
It is an important object of the invention to provide an assembly for retrofitting instruments of established design which will add thereto the capability of generating electrical alarm signals to provide an alert at a remote location of out of limit conditions at the instrument location or for other purposes for which such signals may be used.
It is another important object of the invention to provide a retrofit assembly for adding an alarm feature to instruments lacking such a feature at a fraction of the cost of replacing the instrument with one which includes an alarm feature.
It is still another object of the invention to provide such a retrofit assembly for adding an alarm feature to instruments of established design which is readily installed, does not alter the accuracy of the instrument, and which includes adjustable high and low alarm set points.